B lymphocytes are regulated by multiple environmental stimuli produced during the immune response. Each of these signals may act on different subpopulations of B lymphocytes and/or select responding B cells for particular functions. My laboratory has identified a cDNA for a novel cytokine we call HMW-BCGF (high molecular weight-B cell growth factor). HMW-BCGF induces proliferation and inhibits immunoglobulin secretion by activated human B lymphocytes. Studies using highly purified HMW-BCGF and monoclonal antibodies to both HMW-BCGF and its receptor suggest that this lymphokine plays a role in expanding a subpopulation of memory B cells. Our work in the last two years has revealed that more than one form of HMW-BCGF may exist. We propose to optimize production of recombinant pI 7.8-HMW-BCGF, to examine whether recombinant HMW-BCGF has all the activities we have previously identified for native HMW-BCGF, to analyze the binding of recombinant HMW-BCGF to activated B cells, as well as to cells of other lineages, and to develop a panel of monoclonal antibodies recognizing different epitopes of HMW-BCGF. We will use probes based on the pI 7.8-HMW-BCGF cDNA to examine when HMW- BCGF production occurs in mononuclear cells stimulated in vitro with antigen or mitogens, where HMW-BCGF production occurs in normal human tonsils and spleen, and whether HMW-BCGF production occurs in any normal B cell subpopulations. Finally, we will use monoclonal antibodies and deletion mutants of pI 7.8-HMW-BCGF to determine which portions of the molecule are necessary for binding to B cells and for induction of different B cell functions. These studies will provide new insight into the regulation of human B cell proliferation by HMW-BCGF. They may suggest potential therapeutic uses for HMW-BCGF.